Origin of Muscular Power. 499 



equilibrium with the surrounding air. In the cylinder is a 

 piston fitting air-tight, but moveable without friction. The 

 piston has a certain weight, which we will imagine at first to have 

 balanced the elasticity of the gas underneath it : now let us ap- 

 ply to the gas in the cylinder a certain amount of heat so that 

 its temperature rises ; then the balance between the elasticity of 

 the gas and the weight of the piston will be destroyed. The 

 latter will rise. Here is a certain amount of external work done, 

 and a certain corresponding portion of the added heat disappears. 

 If we now leave the apparatus alone, the rest of the added heat 

 will gradually be abstracted by the surrounding air, and the pis- 

 ton will sink to its old place. It may be observed, in passing, 

 that during this process the heat which had been transmuted 

 into work again appears as heat. We might now propose to 

 ourselves the task of keeping the piston up in the cylinder. To 

 do this the increased temperature of the gas must be maintained. 

 This can only be done by a continual addition of heat to it ; for, 

 according to the conditions of the experiment, it would always 

 be losing heat. Evidently then no more than an exact compen- 

 sation for the loss is needed to maintain the elevation ad infinitum. 

 No more heat will be changed into work, because no more work 

 is done. Suppose, for instance, we produced the warmth neces- 

 sary to maintain the position of the piston by burning carbon, 

 the whole of the heat produced by its burning would now be 

 liberated, and diffused into the medium surrounding the cylinder. 

 We may conceive of a tetanized muscle (like the heated gas in 

 the cylinder) as holding up a weight which would immediately 

 fall if the supply of actual energy were to cease. It is active, 

 but it performs no work, and therefore all the force produced 

 is liberated in the form of heat. 



If we return from this digression to our subject, we shall find 

 that we have another and a last item to add to the total of actual 

 energies which must be provided by force-generating processes 

 in muscles. It is hardly conceivable, on the principles of the 

 mechanical theory of heat, that these processes, even in the case 

 of real muscular work, produce only just the quantity of actual 

 energy needed for the mechanical work in question. It is, on the 

 contrary, tolerably certain that only a part of the actual energy 

 developed by those force-generating processes can be transmuted 

 into mechanical work. This exceedingly probable conclusion, 

 deduced from the most general physical considerations, has been 

 confirmed by experiment. From Heidenhain's* beautiful inves- 

 tigations concerning the connexion of the development of heat 

 with muscular activity, it is possible to estimate, at least ap- 



* Mechanische Leistung vnd Warmeentwicketung im Musket. Leipzig-, 



